1. Field of the Invention
The present invention relates to an optical pickup device for recording, reproducing or erasing information by irradiating a laser beam spot to an optical information recording medium. The present invention also relates to a polarizing beam splitter used in this optical pickup device.
2. Description of the Related Art
In a general optical pickup device in an apparatus for recording and reproducing optical information such as an optical disk unit, grooves are spaced apart from each other at a pitch about 1.6 .mu.m on an optical information recording medium such as magnetooptic disk. In the following description, the optical information recording medium or the magnetooptic disk are respectively called a recording medium or an optical disk. A laser beam is irradiated by an objective lens onto these grooves by changing this laser beam to a spot having a diameter about 1 .mu.m such that information is recorded, reproduced and erased.
Therefore, it is necessary to irradiate the laser beam spot with an accuracy of about 0.1 .mu.m on the information recording grooves spaced apart from each other at a pitch about 1.6 .mu.m, i.e., in the vicinity of the central line of a track while a distance between the recording medium and the objective lens is held and set to be about 4 mm with an accuracy of about 1 .mu.m.
However, the optical disk is rotated at a high speed such as several thousand revolutions per minute to increase a processing speed. Therefore, a position of the track is changed at a high speed at any time by vibrations of an optical disk surface, a core, etc. Accordingly, focusing control for controlling a movement of the objective lens in the direction of an optical axis and tracking control for controlling a spot position in a radial direction of the disk are performed while the position of the track is detected.
In these focusing and tracking controls, light reflected from the optical disk is generally received by divisional light-receiving elements as typically used in detecting methods such as push-pull method, a critical angle method, an astigmatic method and a knife edge method. A difference in divisional output signal between the divisional light-receiving elements is then calculated and the focusing and tracking controls are generally performed by using this difference signal.
In magnetooptic recording and reproducing operations in which information can be erased and overwritten, the information is recorded by vertically magnetizing a magnetic film of the optical disk. An information signal vertically magnetized and recorded to the magnetic film is reproduced as follows. Namely, a linearly polarized laser beam is converged and formed as a spot on the optical disk surface and is changed to elliptically polarized light by rotating a polarizing plane leftward and rightward by a magnetic Kerr effect in accordance with the direction of a residual magnetic field. The elliptically polarized light reflected on the optical disk is separated into a p-polarized light component and an s-polarized light component. The information signal is reproduced by detecting a change in ratio of light amounts of the p and s polarized components.
A polarizing beam splitter is used to separate the reflected light into the p and s polarized light components. The polarizing plane of the reflected light and a polarizing face of the polarizing beam splitter are inclined 45.degree. with respect to each other to accurately detect a signal even when an angle of (leftward or rightward) rotation of the reflected light according to intensity of the residual magnetic field is small. In this case, the amounts of transmitted and reflected lights with respect to the polarizing beam splitter are compared with each other.
Therefore, in a general proposal, the polarizing plane of the reflected light is rotated 45.degree. by inserting a 1/2 wave plate into an optical path before light is incident to the polarizing beam splitter. Otherwise, as shown in Japanese Utility Model Application Laying Open (KOKAI) No. 1-133229, the polarizing beam splitter is rotatably disposed around the optical axis of an optical system and the polarizing surface of this polarizing beam splitter is adjusted such that this polarizing surface is inclined 45.degree. with respect to the polarizing plane of the reflected light.
Each of a tracking control signal, a focusing control signal and a reproducing signal is formed by a difference signal. Accordingly, a C/N ratio is reduced as the amount of incident light is reduced. Therefore, there is a fear that an error signal is caused so that serious errors such as an error in operation of the optical pickup device and an error in reading of information are caused.
The polarizing beam splitter can be constructed by sticking two prisms to each other. For example, the polarizing beam splitter of a cubic type constructed by two rectangular prisms has six abrasive surfaces requiring accurate finishing. The polarizing beam splitter of a type composed of a rectangular prism and a rhombic prism has seven abrasive surfaces requiring accurate finishing. Accordingly, it is necessary to perform an adhering operation in such polarizing beam splitters so that cost of the optical pickup device is increased. The cost of the optical pickup device is further increased if an antireflection film is processed and formed on a total of three surfaces composed of an incident surface and two light emitting surfaces of the polarizing beam splitter.
Further, the amount of materials used for the prisms is large and weight of the optical pickup device is increased together with the cost thereof so that it is difficult to make the optical pickup device compact and light in weight. It is also difficult to reduce the cost and weight of the optical pickup device by disposing a 1/2 wave plate to incline the polarizing surface of the polarizing beam splitter by 45.degree. with respect to the polarizing plane of light. In the proposal for removing the 1/2 wave plate and shown in Japanese Utility Model Application Laying Open (KOKAI) No. 1-133229, the weight of a member for supporting the prisms is increased and processing of the supporting member is complicated so that it is difficult to reduce the cost and weight of the optical pickup device.